AC Small-Signal Analysis

The .AC command calculates the frequency response of a circuit over a range of frequencies.

.AC <sweep type> <points> <start frequency> <end frequency>


sweep type

Must be LIN, OCT, or DEC, as described below.

Sweep type Description
LIN Linear sweep. The frequency is swept linearly from the starting to the ending frequency. The points is the total number of points in the sweep.
DEC Sweep by decades. The frequency is swept logarithmically by decades. The points is the number of points per decade.
OCT Sweep by octaves. The frequency is swept logarithmically by octaves. The points is the number of points per octave.



Specifies the number of points in the sweep, using an integer.

start frequency

Is the starting frequency.

end frequency

Is the final frequency.


The ac small-signal portion of SPICE ( .AC ) computes the ac output variables as a function of frequency. The program first computes the dc operating point of the circuit and determines linearized, small-signal models for all of the nonlinear devices in the circuit. The resultant linear circuit is then analyzed over a user-specified range of frequencies. The desired output of an ac small- signal analysis is usually a transfer function (voltage gain, transimpedance, etc). If the circuit has only one ac input, it is convenient to set that input to unity and zero phase, so that output variables have the same value as the transfer function of the output variable with respect to the input.

All independent voltage and current sources that have AC values are inputs to the circuit. During AC analysis, the only independent sources that have nonzero amplitudes are those using AC specifications. The SIN specification does not count, as it is used only during transient analysis.

Note Note

Note that in order for this analysis to be meaningful, at least one independent source must have been specified with an AC value.


.AC DEC 10 1k 1Meg
.AC DEC 10 1k 100Meg
.AC LIN 100 1 100Hz

See also